Radio direction finding



Patented July 5, 1949 RADIO DIRECTION FINDING Edward D. :Blodgett, .Haddonfield, N. J assignm- .to Radio (Jorporation'of America, a corporation .of Delaware Application 'March 22, 1947, Serial No. 736,432

1 8 Claims.

This invention relates to radio direction ffinders, and more particularly to the reduction or elimination of so-called polarization errors or night effect. such errors are introducedby the response of the antenna system including the horizontal feeders) to wave-components of polarization other than that to which the 'isystem is intended to respond. Ordinarily, the collector elements of the antenna of a direction finder are designed to respond principally to vertically polarized waves. Horizontally polarized energy, if it produces any iresponseat all in such a system, provides erroneous indications.

In general, a horizontally polarized wave com ponent, in travelling parallel to the conductive earth (1. e. as a ground wave) ,is rapidlyattenuated. However, a wave travelling downward cat a large angle to the earth's surface isnot so rapidly attenuated, and it necessarily :has a .horizontally polarized component because the plane of polarization is perpendicular to the direction of travel. The steeper the angleof arrival, the

smaller will be the verticallyrpolarizedcomponent. Thus skywaves, i. e. signals reflected from the Heaviside layer, are apt to provide erronous bearing indications, because the ratio of thehorizontal electric vector to the vertical electric vector is large, and even a small sensitivity (i. e. undesired pick-up) to horizontally polarized energy will produce a responsewhich is of the same order as the desired response to vertically polarized energy,

Various systems have been devised to reduce the above-described horizontal response, such as Adcock antennas using balanced, buried, or shielded horizontal feeders. However, there is always some residual horizontal pick-up, not withstanding extreme care in balancing and shielding the horizontal conductors.

Accordingly, it is the principal object of the present invention to provide a radio direction finder system wherein the necessary horizontal transfer of signal energy from the collector elements to the mixing point, is accomplished without conductors.

The invention will be described with reference to the accompanying drawing,wherein Figure 1 is a plan view of a direction finder system embodying the invention, and

Figure 2 is a schematic block diagram of the circuits in a system according to the present invention.

Refer to Figure 1. Four vertical antennas l,

3, 5 and I are arrangedat-the comers'of asquare- At the base of the antenna 4 is a house 8 containing receiver and transmitter -equipment such as that shown schematically in the upper portion of Figure 2. An ultra-high frequency radiator provided with .a paraboloidal reflector H is provided outside the house 9, ata somewhat lower elevation than the antenna :1. The directive :axis of the reflector H extends toward the center of the square. The antennas 3, 5 and l are similarly provided with houses 13, IE-an'd I respectively containing equipment like that in the house '9, and. with ultra-.highrfrequency radiatorsin'reflectors 19, 2| and 'ZS-e'directed toward the center 'of the square.

A receiving house *25 .is provided at the center of the square, andsupports :a vertical radiator 21 shorter but otherwisesimil-ar'to the vertical -col lector elements I, 3, '5 and I. Also at the house 25 are fourultraehigh ifrequency collectors, in cluding reflectors 229, 13!, 33 and 35 directed respectively toward the radiators at the houses =9, I3, l5 and 17. The central house 25 contains receiving and indicating "equipment such as that shown in the lower portion of Figure 2.

Referring now more particularly to Figure 2, the antenna I is connected to a receiver system including a radio frequency amplifier 35, oscillator 36, first detector 31, intermediate frequency amplifier 39, and a second detector 4!. The circuits are substantially those of a conventional superheterqdyne receiver. The receiver also includes 'an automatic tuning arrangement, comprising a discriminator '43 connected to the output of the I. F. amplifier 39 to control a tuning .motor '45 for adjustment 'of the tuned circuits of the R. F. amplifier'35, the oscillator 36 and the first detector 31.

The output "circuit of the second detector M goes to a modulator, connected to modulate an ultra-high frequency transmitter 49. The transmitter 49 is connected to the radiator in the reflector H. A similar receiver and ultra-high frequency transmitter system is provided at each of the other antenna houses I3, l5 and 11. These are not shown in detail, since they are substantial duplicates of that --at the antenna house 9. Each of the antenna houses may be provided with an independent power supply such as batteries for operation of the radio equipment, so that no electrical conductors are required between the central house 25 and any of the antenna houses.

At the central house 25, the antenna in the refiector 29 is connected toan ultra-high frequency receiver comprising a mixer 5|, intermediate frequency amplifier 53, detector 55, and audio amplifier 51. A similar receiver, comprising the corresponding elements 59, 6|, 63 and 65, is connected to the antenna in the reflector 33. A local oscillator 66 is connected to both mixers 5i and 59.

The outputs of the audio amplifiers 5'3 and G5 are applied in opposition to each other to one stator winding 6'! of a goniometer E9. The goniometer 69 includes also a second stator winding H, orthogonally disposed with respect to the winding 61, and a rotor winding 13 connected to a transducer such as earphones 15. The rotor winding '53 is mechanically coupled to a scale and index M and it for indicating its angular position in terms of hearing.

The stator winding H is connected to the audio output circuits of a second channel comprising receivers identical with those described above, but connected to the antennas in the reflectors l9 and 23 respectively. The common oscillator 66 may be connected to the mixers of the second channel, as well as to the mixers 5i and 53. A radio frequency oscillator Tl, adjustable in frequency, is connected to the central antenna 21.

In the operation of the described system, signals from a distant source whose direction is to be determined will induce voltages in the antennas I, 3, 5 and 1 of substantially equal amplitudes. Suppose the antennas l and 5 lie on a north-south line, and the antennas 3 and 5 on an east-west line. Using the central antenna Zl as a reference point, let the bearing of the distant transmitter be measured clockwise from the north. The voltage induced in the antenna I will lead that induced in the antenna by a phase angle proportional to cosine 6. Similarly the voltage at the antenna 3 will lead that at the antenna 1 by an angle proportional to sine 0.

Since the antennas I, 3, 5 and l are equidistant from the antenna 27, the signal from the local oscillator 11 will induce equal voltages, in phase with each other, in the four collectors. The oscillator TI is adjusted to operate at a frequency differing slightly (by 500 cycles per second, for example) from the frequency of the arriving signal Whose direction is to be determined. Both the voltages induced in the collector antenna l are amplified, converted to intermediate frequency, further amplified, and then demodulated by the detector 4|.

The output of the detector 41 includes a beat component, of a frequency equal to the difference in frequency (e. g. 500 cycles) between the signal derived from the distant source and that provided by the local oscillator ll. This low frequency component is applied to the modulator 41, impressed on the ultra-high frequency carrier of the transmitter 49, and sent to the central house. The frequency of operation of the ultrahigh frequency link may be of the order of 3000 megacycles per second.

The voltage induced in the collector l by the locally generated radio frequency signal from the antenna 2'! may be several times the amplitude of the voltage induced by the remote source. The same relationship holds between the corresponding intermediate frequency voltages at the output of the amplifier 39. Thus the automatic frequency control system, comprising the discriminator 43 and the tuning motor 45, is controlled entirely by the output of the local oscillator 11.

Suppose the intermediate frequency to which the amplifier 39 is tuned to be 800 kilocycles per second. If the oscillator l? is adjusted to operate at say megacycles per second, the oscillator 36 should be tuned to 10.8 megacycles to provide 800 kilocycles output from the first detector 31 to the amplifier 39. If the oscillator 36 is operating at some frequency other than 10.8 megacycles, the output of the detector 32' and the am plifier 39 will have a frequency higher or lower than 800 kilocycles, causing the discriminator 43 to provide output of such polarity as to make the tuning motor 45 drive the control of the oscillator 36 toward 10.8 megacycles. The tuning controls of the radio frequency amplifier 35 and the detector 3'! are adjusted similarly.

Signals from other distant sources, of frequencies substantially difierent from 10 megacycles, are rejected by the tuned circuits of the amplifier 35 and the detector 3?. Other signals, close enough in frequency to pass the tuned circuits and provide output to the discriminator 43, still cannot affect the tuning because they are much lower in amplitude than that derived from the oscillator 17. When operation is desired on a different frequency, for example twelve megacycles, the oscillator TI is adjusted to that frequency. The discriminator 43 responds, causing the tuning of the receiver elements 35, 36. and 31 to remain in step.

The above-described operation is duplicated by the receiver systems at the other collector antennas 3, 5 and i. The modulated ultra high-frequency carriers are received at the central house. The ultra-high frequency oscillator 66 operates at, for example, 3032 megacycles per second. The mixer 5| produces an output of 32 megacycles frequency, modulated like the ultra-high frequency carrier. The amplifier 53, tuned to 32 megacycles, amplifies the output of the mixer 5|, which is then demodulated by the detector 55.

The output of the detector is substantially the same as that of the detector 4! at the house 9. Likewise, the output of the detector 63 is the same as that of the corresponding detector at the house l5. The phase relationship between these two low frequency voltages is the same as that between the high frequency carriers received at 7 the antennas l and 3 from the distant source.

The outputs of the detectors 55 and 63 are amplified by the amplifiers 51 and 65 respectively, and applied in opposition to each other to the goniometer winding 61. The amplitude of the resultant voltage across the winding 61 is substantially proportional to the difference in phase of the differentially applied components, i. e. cos 0. The amplitude of the voltage across the goniometer winding H is similarly proportional to sine 0. The two resultant voltages, and hence the fields produced by the windings 61 and 'H, are substantially in time phase with each other, and this phase relationship does not vary with variation in the bearing angle 0. The two fields are in space quadrature, however, so they combine to provide a resultant field whose direction, with respect to the axis of the winding 61, corresponds to the bearing angle 0. This direction is determined, in the present example. by rotating the goniometer coil 13 to the position providing minimum output to the headphones 15. The axis of the coil 13 is then at right angles to the resultant field of the stator coils 61 and 1 I, and its position indicates the bearing angle.

In the above-described embodiment of the invention, the local oscillator 11 performs the double function of providing a beating signal for the receivers at the antenna houses, and controlling the tuning of said receivers. It will said collector antennas, and means for modulating each of said transmitters with the output of the respective receiver, further receiver means remote from said collector antennas and. responsive respectively to said transmitters to reproduce the respective outputs of said first-mentioned receivers, and means indicating the phase relationship between said reproduced outputs.

8. A radio direction finder system including at least two collector antennas, receiver means at and connected to each of said antennas, a local radiation source providing continuous wave energy of a frequency differing by a predetermined amount from the frequency of a signal whose direction of arrival is to be determined, 15

whereby each of said receivers provides an out:- put at a frequency corresponding to said difference in frequency; a transmitter at each of said collector antennas, and means for modulating each of said transmitters with the output of the respective receiver, further receiver means remote from said collector antennas and responsive respectively to said transmitters to reproduce the respective outputs of said first-mentioned receivers, and means indicating the phase relationship between said reproduced outputs.

EDWARD D. BLODGETI.

No references cited.

"be apparent however, that the local oscillator may be used solely for controlling the tuning of the receivers, by providing a separate control receiver at each antenna house, responsive to the central oscillator for controlling the tun-ing of the main receiver. In this event, it is preferable to operate the control oscillator at a frequency widely different from that of the wave whose direction of arrival is to be indicated. The goniometer 69 may then be operated at radio frequency or intermediate frequency instead of audio frequency. As in the system of Figure 2, pick-up of horizontally polarized energy is substantially avoided by the elimination of horizontal feeders from the collector antennas.

I claim as my invention:

1. A radio direction finder system including a plurality of spaced collector antennas, receiver means at and connected to each of said antennas, a local radiation source equidistant from said antennas and providing continuous wave energy of a frequency differing by a predetermined amount from the frequency of a signal whose direction of arrival is to be determined, means responsive to the frequency of said local radiation source to tune said receivers to respond to said signal; a radio transmitter at each of said collector antennas, and means for modulating each of said transmitters with the output of the respective receiver, further receiver means remote from said collector antennas and responsive respectively to said transmitters to reproduce the respective outputs of said first-mentioned receivers, and means indicating the phase relationship between said reproduced outputs.

2. A radio direction finder system including a plurality of spaced collector antennas, local oscillator means adjustable to a frequency differing by a predetermined amount from that of an arriving wave whose direction is to be determined, a radiator at a point equidistant from said collector antennas connected to said local oscillator, receiver means at each of said antennas including variably tunable circuits, means responsive to the frequency of said local oscillator to adjust the tuning of said circuits to a frequency substantially the same as that of said local oscillator and demodulator means, radio transmitter means near each of said collector antennas and means for modulating each of said transmitters respectively with the output of the corresponding demodulator, receiver means remote from said collectors and responsive respectively to said transmitters to reproduce the respective outputs of said demodulators, and means indicating the phase relationship between said reproduced outputs.

3. A radio direction finder system including a plurality of spaced collector antennas, receiver means at and connected to each of said antennas, a local radiation source equidistant from said antennas and providing continuous wave energy of a frequency differing by a predetermined amount from the frequency of a signal whose direction of arrival is to be determined, whereby each of said receivers provides an output at a frequency corresponding to said difference in frequency; a radio transmitter at each of said collector antennas, and means for modulating each of said transmitters with the output of the respective receiver, further receiver means remote from said collector antennas and responsive respectively to said transmitters to reproduce the respective outputs of said first-men- "6 tioned receivers, and means indicating the phase relationship between said reproduced outputs.

4. A radio direction finder system including a plurality of spaced collector antennas, receiver means at and connected to each ofsaid antennas, a local radiation source equidistant from said antennas and providing continuous wave energy of a frequency differing "by a predetermined amount from the frequency of a signal whose direction of arrival is 'to be determined, means responsive to the frequency of said local radiation source to tune said receivers to substantially the same frequency as that of said local source, whereby each of said receivers provides an output at'a frequency corresponding to said difference in frequency between said signal and said local source; a radio transmitter at each of said collector antennas, and means for modulating each of said transmitters with the output of the respective receiver, further receiver means remote from said collector antennas and responsive respectively to said transmitters to reproduce the respective outputs of said first-mentioned receivers, and means indicating the phase relationship between said reproduced outputs.

5. A radio direction finder system including a plurality of spaced collector antennas, local oscillator means and a radiator therefor at a point equidistant from said collector antennas, receiver means at each of said antennas including variably tunable circuits, and means responsive to the frequency of said local oscillator means to adjust the tuning of said circuits to a frequency substantially the same as that of said local oscillator, radio transmitter means near each of said collector antennas and means for modulating each of said transmitters respectively with the output of the corresponding receiver, further receiver means remote from said collectors and responsive respectively to said transmitters to reproduce the respective outputs of said receivers at said collectors, and means indicating the phase relationship between said reproduced outputs.

6. A radio direction finder system including a plurality of spaced collector antennas, local oscillator means adjustable to a frequency differing by a predetermined amount from that of an arriving wave whose direction is to be determined, a radiator at a point equidistant from said collector antennas connected to said local oscillator, receiver means at each of said antennas including variably tunable circuits, means responsive to the frequency of said local oscillator to adjust the tuning of said circuits to a frequency substantially the same as that of said arriving wave, radio transmitter means near each of said collector antennas and means for modulating each of said transmitters respectively with the output of the corresponding demodulator, receiver means remote from said collectors and responsive respectively to said transmitters to reproduce the respective outputs of said demodulators, and means indicating the phase relationship between said reproduced outputs.

7. A radio direction finder system including at least two spaced collector antennas, receiver means at and connected to each of said antennas, a local radiation source providing continuous wave energy of a frequency differing by a predetermined amount from the frequency of a signal whose direction of arrival is to be determined, means responsive to the frequency of said local radiation source to tune said receivers to respond to said signal; a transmitter at each of 

